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From almost the moment their discovery was announced, everyone agreed that the first sighting of gravitational waves was going to win a Nobel Prize. The only questions were when and who would receive the honor. Both of those questions have now been answered. When is now, and who turned out to be three individuals who contributed to the project in very different ways.

Caltech's Kip Thorne, a theoretician who made sure we knew what a gravitational wave would look like when we saw it, was one honoree. He was joined by Rainer Weiss, an MIT scientist who helped build some of the first prototype detectors that would eventually inspire the LIGO design, and Barry Barish, another Caltech physicist who was put in charge of the LIGO collaboration and became instrumental in ensuring that the hardware was built and that a large international collaboration was present to operate it and analyze the results.

While LIGO was a stunning success, its history suggests that there were countless ways it and the entire field of gravitational wave physics might have failed. And those ways all lead back to the very person whose work suggested that space-time itself could experience ripples.

Is there anything out there?

Gravitational waves are changes in the fabric of space caused by the acceleration of an object. All objects produce them, but only the most massive can make a wave that can possibly be detected. Their existence was a consequence of Einstein's theory of relativity, although they weren't an obvious outcome of his equations when he first proposed them in 1915. In fact, Einstein wrote a paper in 1936 in which he attempted to demonstrate that gravitational waves could not possibly exist. A peer reviewer recognized some mistakes in his math and rejected the paper.

Einstein and further researchers later went on to show that gravitational waves could potentially exist, but the math was complicated enough that it took until the 1950s for a rigorous proof to be devised. With that, we knew they could exist, but there was serious doubt that they could ever be detected. At that point, black holes themselves were still a theoretical construct, and many in the physics community doubted they actually existed, too.

Further Reading

But experiments to detect waves started in the 1960s, and, by the early 1970s, theoretical physicists (including Kip Thorne) had started to work out the behavior of black holes. This would form the foundation of the simulations that would tell us what a black hole collision would look like from a gravitational wave perspective.

Enlarge/ The top two graphs show the predicted signal of gravitational waves, based on Kip Thorne's work, compared to what was seen at the two LIGO detectors.

Meanwhile, a physicist named Joseph Weber claimed that he had built a detector that successfully picked up gravitational waves. Attempts to replicate the results, however, failed. This might normally have put a damper on the field, but the astronomy community bailed out the physicists. Joseph Taylor and Russell Hulse tracked the motion of a pair of pulsars orbiting each other at a close distance. The results showed that the pulsars were losing energy at the rate predicted for the production of gravitational waves, work that later won the duo a Nobel Prize and eliminated any lingering doubt that the waves existed.

Detectors that detected nothing

Although Weber's detector design didn't work out, other ideas for detecting gravitational waves were also floating around. LIGO's design, called an interferometer, can trace its heritage back to the Soviet physicists (M.E. Gertsenshtein and V.I. Pustovoit) who did some work on it in the early 1960s. The new laureate Rainer Weiss got involved later that decade when he designed laser-based hardware with sensitivity that, theoretically at least, was limited only by the quantum nature of light. But, critically, Weiss went on to consider how to deal with all the things that could also limit the sensitivity. That includes seismic noise, stray heat, vibrations of the atoms in the mirrors that reflect the lasers, problems with the laser-generating hardware itself, and more.

Weiss' work inspired groups in Germany and Scotland to build prototype detectors up to 10 meters in length. (Thorne later convinced Caltech to hire one of the Scottish team members to start a similar project there.) While nowhere large enough to detect gravitational waves, their detectors provided important practical experience.

At this point, the US government stepped in, in the form of the National Science Foundation. It commissioned Weiss to do a design study for something big enough to actually detect gravitational waves based on this design. The results were a detector with arms five kilometers long.

NSF liked the idea and asked MIT and Caltech to join forces, bringing Weiss and Thorne onto the same team to create the nucleus of LIGO. But for several years, the only thing the scientists produced were a variety of design studies; attempts to actually build anything stalled. At this point, the NSF stepped in again and got Barry Barish named to head the project.

Further Reading

Barish had experience with large international particle physics collaborations (he'd later leave to head up the International Linear Collider effort), and he reshaped LIGO to reflect that. The German and Scottish teams were invited to join, and they contributed key technology to the project. Other international research groups soon followed. Critically, Barish came up with the idea of building a detector that probably wouldn't work but would provide the necessary experience and technology to build one that would. Amazingly, the NSF took the risk and funded not one, but two detectors that probably would never detect anything.

By 1998, the facilities were built, and the instruments were installed in 2002. From 2002 to 2010, this version of LIGO took data but saw nothing. However, it laid the groundwork for several years of upgrades that led to Advanced LIGO, which was ready for operation in 2015.

Within three weeks, it spotted its first gravitational wave event.

A new era

Entire books could be written about all the steps that LIGO takes to limit the noise that is a constant feature of its data, including the fact that two detectors were built 3,000 kilometers apart in order to ensure that they experience different noise. Countless accelerometers, seismometers, and microphones let the LIGO team identify sources of noise that can't be eliminated.

The results are measurements that are sensitive enough to pick up changes in distance many times smaller than an atom's nucleus and can detect the flexing of the crust caused by the orbit of the Moon. LIGO can even pick up the motion of the mirrors caused by its lasers bouncing photons off them. And yet, astonishingly, the team plans to increase the sensitivity through further upgrades.

Other books could be written about the exotic sources of the gravitational waves. The gravitational wave events recorded in LIGO are actually the first direct indication of the existence of black holes. It's also expected to be able to pick up the merger of neutron stars, which should produce a similar (though weaker) pattern of waves. The hope is also that we'll see rare single waves caused by things like an asymmetrical supernova or a magnetic burst that reshapes the surface of a magnetar.

But it's probably best to think of the achievement this way. For most of our history, humanity's exploration of the cosmos has relied on its ability to sense light, a capacity that evolved billions of years ago. But in the past decade, we've added two radically different ways of sensing the Universe. One is the IceCube cosmic neutrino detector; the second is LIGO.